Electrostatic coating method

ABSTRACT

A CONDUCTIVE SENSITIZED BINDER COMPRISING A POLYMERIZABLE BINDER MATERIAL SUCH AS POLYVINYL ALCOHOL AND A METALLIC SALT SUCH AS AMMONIUM DICHROMATE DISSOLVED IN A WATER SOLUBLE ALCOHOL AND WATER IS APPLIED TO A NONCONDUCTIVE SURFACE OF A TELEVISION PICTURE TUBE. ELECTROSTATICALLY CHARGED PHOSPHOR PARTICLES ARE PLACED ADJACENT THE TUBE SURFACE. THE CONDUCTIVE COATING OF SENSITIZED BINDER IS MAINTAINED AT PARTICLE-ATTRACTING POTENTIAL. PARTICLES ARE ELECTROSTATICALLY DEPOSITED ON THE CONDUCTIVE SENSITIZED BINDER, WHICH IS ACTIVATED, AFFIXING THE PARTICLES. ONE METHOD OF DEPOSITING PHOSPHOR PARTICLES INCLUDES PROJECTING THE PARTICLES FROM A FLUIDIZED BED TOWARD THE   CONDUCTIVELY COATED TUBE SURFACE AND ESTABLISHING AN ELECTROSTATIC FIELD TO THE CONDUCTIVE COATING FROM AN ELECTRODE TO ELECTROSTATICALLY CHARGE AND DEPOSIT THE PHOSPHOR PARTICLES. THE TELEVISION PICTURE TUBE AND THE ELECTRODE MAY BE MOVED RELATIVE TO EACH OTHER DURING DEPOSITION.

June 27, 1972 L. SPILLER ETA!- 3,672,927

ELECTROSTATIC COATING METHOD Original Filed Feb. 28, 1966 INVENTOIZS ifi z// 553;; J 52-1 United States Patent 01 ace 3,672,927 Patented June27, 1972 3,672,927 ELECTROSTATIC COATING METHOD Lester L. Spiller andStephen J. Smith, Indianapolis, Ind.,

assignors to Ransburg Electra-Coating Corp., Indianapolis, Ind.

Continuation of application Ser. No. 530,473, Feb. 28, 1966. Thisapplication Oct. 29, 1969, Ser. No. 873,761 Int. Cl. B05b 5/00; B44dI/095 U.S. Cl. 117-17 Claims ABSTRACT OF THE DISCLOSURE -A conductivesensitized binder comprising a polymerizable binder material such aspolyvinyl alcohol and a metallic salt such as ammonium dichromatedissolved in a water soluble alcohol and water is applied to anonconductive surface of a television picture tube. Electrostaticallycharged phosphor particles are placed adjacent the tube surface. Theconductive coating of sensitized binder is maintained atparticle-attracting potential. Particles are electrostatically depositedon the conductive sensitized binder, which is activated, aiiixing theparticles. One method of depositing phosphor particles includesprojecting the particles from a fluidized bed toward the conductivelycoated tube surface and establishing an electrostatic field to theconductive coating from an electrode to electrostatically charge anddeposit the phosphor particles. The television picture tube and theelectrode may be moved relative to each other during deposition.

This invention is concerned with methods for coating television picturetubes with phosphors by the use of electrostatic forces. Thisapplication is a continuation of prior application Ser. No. 530,473filed Feb. 28, 1966, now abandoned.

It has heretofore been the practice in the production of picture tubesfor television receivers to apply phosphors either as dry powders or assediments from liquid suspensions to an adherent layer on the glasssurface of the tube face. In the case of those tubes to be used in colortelevision receivers, the process has involved the application of abinder containing a sensitizing agent to the inside surface of the tubeface, and then dry particles of the phosphor are dusted thereover toproduce a thin power layer. The thin powder layer over the sensitizedbinder layer is then exposed to a suitable image of an array of dots,and the material within the area of the dots is polymerized, oraffixed,onto the tube face. The remaining material, being water soluble, is thenremoved by a water rinse and a grid of isolated dots is thus formed overthe face surface. The process is then repeated again using thesensitized binder and a different phosphor with the polymerization ofdots of the material located in different areas than the first set ofdots. The process is repeated for a third time using the sensitizedbinder and still a third different phosphor with the polymerization ofthe dots of the material being located in still different areas to thusproduce a finished coating of the phosphors, preferably one particledeep, over the face surface being coated. Since the three specializedgroups of dots are each of a phosphor designed to emit, when excited, adifferent primary color, the complete surface is thus adapted toreproduce the entire color spectrum.

Such methods are extremely expensive. The necessity for dusting thephosphors over the tube face is time consuming and thus yields lowproduction rates. Further, such hand operations create a high incidenceof tube faces which have to be rejected by reason of nonuniformcoatings. The phosphors employed in the coating are extremely expensive,and with the high incidence of rejects, substantial quanitities of theseexpensive phosphors are thus wasted.

It is an object of this invention to provide a method for coatingtelevision receiver picture tubes which will overcome the difiicultiesand disadvantages discussed above. More specifically, it is an object ofthis invention to provide a method which will electrostatically deposita coating of phosphor powders on television picture tube faces, whichwill direct said phosphor powders to and deposit them on a tube face byelectrostatic forces, which will provide a uniform coating on thesurface to be coated, and which will be efficient in operation. Afurther object of the invention is to provide a method of coating suchtube faces in which the sensitized binder for adhering the phosphorsonto the tube face will serve as an agent for supplying surfaceconductivity to said face for reception of the electrostatically chargedphosphor particles.

According to one form of the invention for coating color television tubefaces, the uncoated face has a sensitized binder applied in a uniformlayer to its surface to be coated. The layer of the sensitized binder isplaced in contact with ground, and the tube face is disposed over afluidized bed of the phosphor to be deposited. A charging electrode isinterposed between the surface of the bed and the tube face, and aplurality of charging members are carried in said bed. Application of apotential difference between the electrode and charging members and theface electrostatically charges the phosphor particles and directs themonto the sensitized layer as said particles are projected upwardly fromthe bed. Conveniently, during deposition the electrode and the tube faceare moved relative to each other.

After the phosphor particles have been deposited, light is projectedonto the coated surface through a dotted screen to activate thesensitizing agent, polymerize the binder, and thus cause the depositedphosphor particles to be bound at the dots onto the tube surface. Aftersuch polymerization, the nonpolymerized binder and phosphor particles atthe areas other than the dots are removed from the surface. Theapplication of the sensitized binder, phosphor powder deposition, andpolymerization are repeated twice again, each time using a differentphosphor and different dot locations. This results in a finished coatingconsisting of a triple array of nonoverlapping dots uniformly disposedover the surface of the tube; each dot having uniform thickness ofphosphor.

Other objects and features of the invention will become apparent fromthe more detailed description which follows and from the accompanyingdrawings in which:

FIG. 1 is a vertical section through an apparatus with which theinvention can be used; and

FIG. 2 is a plan view of the electrode assembly shown in FIG. 1.

In the production of picture tubes for color television receivers, acoating of phosphors is applied to the tubes. Said coating, which issubstantially only one particle thick, is formed of phosphors having aparticle size of about three microns which are adapted to producevisible red, blue, and green phosphorus when bombarded by electrons.

To produce such a coating, we clean the tube face 10 and apply thereto,as by spraying, brushing, or the like, a polar binder solutioncontaining a water soluble lightactivatable sensitizing agent.Desirably, said solution comprises about 3% of polyvinyl alcohol andabout /2 of a chromic salt, such as ammonium dichromate, dissolved inisopropyl alcohol and water in a 1:1 ratio. The polyvinyl alochol servesas a binding agent for the phosphors to be deposited. The chromic salt,in addition to providing light sensitivity, acts in combination with thepolar solvents, isopropyl alcohol and water, to increase theconductivity of the solution to thereby give the surface to be coated asurface resistivity in the range of from about 10 to 3 about ohms persquare. After the solution has been applied to the inside surface of thetube face, it is allowed to dry to a tacky condition. The tube face isthen ready to have the first phosphor applied thereto and is placed inan apparatus for coating, for example as illustrated in FIG. 1.

As shown, the apparatus for coating comprises a chamber 11 formed froman electricity insulating material and comprises a plurality ofinterconnected side walls 12 projecting upwardly from a bottom wall 13.Conveniently, a jacket 14 formed from electrically insulating materialand supported on a plurality of feet 15 extends around the chamber 11Apair of arms 16 extend across the jacket 15 to support the chamber 11 inspaced relation thereto whereby said jacket catches any of the phosphorpowder that flows over the edges of tht chamber. As shown, the upper endof the jacket 14 is spaced slightly below the upper ends of the chamberwalls 12, and the lower end 17 of said jacket has a frustoconicalconfiguration for collecting the powders in the jacket. Conveniently, avacuum pump 18 is mounted at the bottom of the jacket for conveying thenondeposited powder from the jacket to a collector reservoir 19.

A pair of arms 20 project upwardly from the jacket 14 and extendtransversely across the chamber 11. Said arms support a rotationaldriving motor 21 to which a hanger 22 is removably connected. Saidhanger has outwardly projecting arms 23 joined at their outer ends tothe upper face of a ring 24 having an upturned lip 25 at its innercircumference disposed in contact with the conductive binder layer onthe tube face 10 for supporting said tube face in an elevated positionabout the upper end of the chamber 11. An annular shield 26 formed froman electrically insulating material is connected to the lower face ofthe ring 24 and projects laterally outwardly over the chamber 11 andjacket 14. The hanger 22, ring 24, and arms are formed from anelectrically conductive mate rial and are connected to ground, as at 27,to thus ground the sensitized binder layer on the tube face 10. Thus,upon actuation of the motor 21, the grounded tube face 10 will berotated with respect to the chamber 11.

An air permeable plate 28 formed from an electrically insulatingmaterial is mounted in the chamber 11 above the bottom wall 13 andsupports the phosphor particles 30 to be deposited. Air is introducedinto the compartment 32 between the bottom wall 13 and plate 28 throughan air inlet 33 connected to a source of pressurized air 34. The airmoves upwardly from the compartment 32 through the plate 28 to maintainthe phosphor particles 30 in suspension immediately above said plate andthus provide a fluidized bed of said particles. By increasing the flowrate at the inlet 33, the particles will billow upwardly from the bed inthe chamber toward the tube face 10 and thus form a cloud having aparticle density substantially less than the bed; the degree ofbillowing being dependent upon the flow rate at the inlet 33.

An electrode assembly is mounted on the plate 28 and comprises a base 36supporting an upwardly projecting spindle 37. The upper end of thespindle is connected to a charging head 38 conveniently having the samegeneral shape and size as the surface to be coated and comprising aplurality of annuluses 39 interconnected by a wire grid 40. A pluralityof electrode points 42 project upwardly from the head 38 toward the tubeface with their ends substantially equally spaced from said face.Conveniently, a plurality of pins 43 also be mounted in the plate 28 toproject thereabove into the fluidized powder bed. The electrode assemblyand the pins 43, when the latter are employed, are formed from anelectrically conductive material and are connected to a high voltagesource 44 for electrostatically charging the phosphor particles.

When the cloud of phosphor particles has billowed around the electrodehead above the points 42, the tube face 10 is rotated and the voltage isapplied to the head; preferably in an interrupted manner in twosuccessive applications. The voltage is negative and provides an averagevoltage gradient between the points 42 and the tube face of at least 5kilovolts per inch. With the tube face being grounded and the electrodebeing charged, the phosphor particles projected upwardly from the bedaround the head and in the field extending between the head and tubeface acquire an electrostatic charge an are thus attracted to anddeposited onto the tube face. When the pins 43 are employed, they chargethe adjacent particles in the bed and help to direct the particles tothe tube face to help insure a uniform coating thereon. With the headrotating, a uniform voltage gradient will be established between thehead and tube face to insure a uniform deposition of the phosphorparticles onto the tube face. Any of the upwardly projected phosphorparticles which are not deposited will flow over into the space betweenthe chamber 11 and jacket 14 and can be reclaimed in the reservoir 19.The shield 26, being nonconductive, will not have any of the particlesdeposited on it and will serve to direct the nondeposited particles intothe space between the chamber and jacket.

An example of the deposition step of our invention may be described asfollows: The inside of an 11 inch tube face was cleaned and then washedwith the sensitized binder, after which it was allowed to dry for sixminutes at a relative humidity of 40% and a room temperature of F. untilit was in a tacky condition. The face was then mounted on the hanger 22over the deposition chamber 11, which chamber had a length and width of12 inches and an overall height of 15 inches. The plate 28 containingfour equally spaced pins 43 was located 5 inches above the chamberbottom wall, and the electrode head 38 was 5 /2 inches above the plate28. This provided a 6 inch spacing between the electrode head and tubeface. The tube face was rotated at r.p.m., and air was introduced intothe compartment 32 at the rate of 5 c.f.m. When the phosphor particleshad billowed around the head 38, the head and pins were charged at 60kv. in two successive intervals of one second each with about a onesecond interval between each charging interval. This produced a uniformcoating over the insidesurface of the tube face, and said face wasremoved for subsequent processing.

After the phosphor particles have been deposited, the tube face 10 isremoved from the coating chamber, and a beam of light is projectedthrough a dotted image-producing screen and onto the coated surface. Thelight passing through said screen will polymerize the polyvinyl alcoholto thus retain or afiix the phosphor particles as a coating on the tubeface within the dotted areas not masked by the screen. Afterpolymerization, the coating is rinsed with water, and the nonpolymerizedpolyvinyl alcohol and the phosphor particles not within the dotted areasare rinsed away.

After the dotted coating has dried, a second layer of the bindersolution is applied thereover. The deposition process is repeated in themanner perviously described using a phosphor having a different colorphosphorescence. After deposition, the polyvinyl alcohol in the bindersolution is polymerized by projecting a beam of light through a seconddotted image-producing screen and onto the coated surface. Said screenhas its dotted pattern offset from the dotted pattern of the firstscreen whereby the second layer of binder solution will be polymerizedin dotted areas adjacent the polymerized dots of the first layer ofsolution. In this manner, a double array of nonoverlapping dots of thefirst and second phosphors will be aflixed to the tube face.

After this second coating has dried, the sequence is again repeatedusing a third layer of binder solution and depositing a third phosphorhaving still a different color phosphorescence. After deposition of thethird phosphor, the polyvinyl alcohol in the third layer of bindersolution is polymerized by projecting a beam of light through a thirddotted image-producing screen and onto the coated surface. The dotpattern in the third screen is offset from the dot patterns in the firstand second screens whereby the third layer of binder solution will bepolymerized in dotted areas adjacent the polymerized dots of the firstand second layers of solution. In this manner, a finished coating isproduced having a triple array of nonoverlapping dots of the first,second, and third phosphors aflixed to the tube face.

As explained above, the production of picture tubes for color televisionreceivers requires the use of three different phosphors with theirparticles afiixed to the tube face in a triple array of adjacentnonoverlapping dots. In the production of picture tubes for black andwhite television receivers only one phosphor need be employed, and itdoes not have to be deposited in any dotted pattern on the tube face,but only in a uniform thin coating over said tube face. Therefore, asingle application of the binder solution and the electrostaticdeposition of only one phosphor need be employed in the production ofsuch picture tubes. Further, because such tubes do not require thephosphor to be in a dotted pattern, the entire deposited coating issubjected to light projected onto it directly, and no image-producingscreens need be employed.

While we have described the tube face as being rotated with respect tothe fixed electrode assembly, it is to be understood, of course, thatthe tube face may be fixed while the electrode assembly is rotated withrespect to it, or in some applications it may even be desirable toretain both the tube face and electrode fixed during deposition.

For convenience, reference has been made herein to powder particlessuspended in air. However, said particles may be suspended in gaseousmedium other than air, and air is intended to include other suitablegases.

We claim:

1. A method of applying a coating of phosphor particles to a televisionpicture tube face, comprising the steps of applying a conductive coatingof a binder and a sensitizing agent to a nonconductive surface of thetube face to be coated, the coating formed from a solution of about 3%polyvinyl alcohol and about /2 of chromic salt dissolved in isopropylalcohol and water in a 1:1 ratio, establishing an adjoining relationshipbetween electrostatically charged phosphor particles and the surface tobe coated, maintaining the conductive coating on the surface at aparticle-attracting electrical potential to effect deposition of saidparticles on said coating, and activating said coating to affix thedeposited particles onto the tube face surface.

2. The invention as set forth in claim 1 in which said coating is driedto a tacky condition prior to establishing an adjoining relationshipbetween the particles and the surface to be coated.

3. The invention as set forth in claim 1 in which said coating of abinder and a sensitizing agent provides said tube face with a surfaceresistivity in the range of to about 10 ohms per square.

4. A method of applying a coating of phosphor particles to anonconductive television picture tube face, comprising the steps ofapplying a conductive coating formed from a polymerizable bindermaterial and a metallic salt dissolved in a water soluble alcohol andwater to the surface of the tube face to be coated, projecting a cloudof said particles suspended in gas toward said tube face, establishingan electrostatic field to said coating, imparting an electrostaticcharge to said particles and effecting deposition of said particles onsaid coating, polymerizing the binder of said coating afiixing thedeposited particles onto the tube face surface, and removing thenonpolymerized binder and non-aflixed particles.

5. The invention as set forth in claim 4 in which said binder islight-activatable, and said activating step comprises subjecting thecoated surface of the tube face to a source of light to polymerize saidbinder.

6. The invention as set forth in claim 4 in which the step of removingthe nonactivated binder and nonaflixed particles comprises flushing thecoated tube face surface with water.

7. The invention as set forth in claim 4 in which said electrostaticfield is interrupted at least once while said particles are beingprojected toward the tube face.

8. The invention as set forth in claim 4 in which said coating providessaid surface of the tube face with a surface resistivity in the range offrom about 10 to about 10 ohms per square, and there is a voltagegradient between said electrode means and tube face of at least 5kilovolts per inch.

9. The invention as set forth in claim 4 with the addition thatelectrode means used to establish the electrostatic field to the coatingand tube face are moved relative to another while said particles arebeing deposited.

10. The method as set forth in claim 4 wherein the coating is dried to atacky condition prior to projecting a cloud of particles toward the tubeface.

References Cited UNITED STATES PATENTS Re. 22,419 1/1944 Smysen 117173,327,685 6/ 1967 Heyl et al. 117-17 3,513,011 5/1970 Miller 117-172,837,429 6/ 8 Whiting 11733.5 2,996,380 8/1961 Evans 11733.5 3,140,1767/1964 Hoffman 11733.5 2,940,864 6/ 1960 Watson 11717 3,000,752 9/1961Jackson 117-17 3,019,126 1/ 1962 Batholomew 1-1717 3,248,253 4/1966Barford et al. 11721 3,269,838 8/1966 Saulnier 11733.5 3,317,319 5/ 1967Maynud 1-1733.5 3,323,933 6/1967 Barford et al. 117-17 3,377,183 4/1968Hurt et al. 117-17.5

FOREIGN PATENTS 592,860 10/ 1947 Great Britain.

WILLIAM D. MARTIN, Primary Examiner M. SOFOELEOUS, Assistant ExaminerU.'S. Cl. X.R.

1-17--33.5 E, 33.5 C, 33.5 CM, Digest 6, 33

